[X86] Use MOVZX16rr8/MOVZXrm8 when extending input for i8 udivrem.

We can rely on X86FixupBWInsts to turn these into MOVZX32. This
simplifies a follow up commit to use MOVSX for i8 sdivrem with
a late optimization to use CBW when register allocation works out.

llvm-svn: 371242